Microwave ovens having automatic cooking control features are widely used because they make cooking more convenient and simple. Various types of sensors have been used in prior art microwave ovens that have automatic cooking functions.
The types of sensors which previously have been used in such microwave ovens are sensors for detecting temperature, sensors for detecting the humidity level, sensors for detecting vapor generated by cooking, and sensors for detecting the weight of food inside of the oven. However, microwave ovens that employ only these types of sensors have a limit in their ability to detect precise cooking conditions.
For example, even though various automatic cooking control functions are available using humidity detection sensors (especially the absolute humidity detection sensors that are widely used for automatic cooking control of microwave ovens), these prior art sensors cause problems when cooking under low heat. Specifically, when warming up food or thawing meat or fish, it is difficult to properly control cooking based on humidity detection because the quantity of moisture generated throughout the entire cooking process is very small.
Furthermore, when using a single container to successively thaw multiple pieces of meat or fish, prior art ovens that employ a humidity sensor often malfunction due to vapor being generated by the premature boiling of residual water left over from the previous thawing operation. As these vapors are not indicative of the current thawing process, they falsely indicate the status of the current thawing process. In order to reduce the frequency of this problem, manufacturers have attempted to explain in the owners' manual the necessity of fully cleaning and drying a container before using that container to thaw a new piece of food. Such efforts, however, may have the undesirable side effect of making microwave thawing cumbersome and inconvenient.
Other prior art microwave ovens employ sensors for directly detecting the temperature of the food being cooked. Despite the advantages offered by the precise detection of the cooking conditions, there are several disadvantages to the temperature sensor directly contacting the food, including the possibility of giving users a feeling of uncleanliness, hygiene concerns and the inconvenience of manipulating the sensor into the food.
Accordingly, a remote method for sensing the food temperature is desired. One example of such a prior art technique uses pyroelectric infrared sensors. Using pyroelectric infrared sensors for continuous temperature sensing, however, requires a mechanically driven chopper for polarization. This complicates the construction and increases manufacturing costs. Additionally, such devices are difficult to apply in practical applications.